This invention relates to improved polyurethane hydrogels and a method for the production thereof. More particularly, this invention relates to highly stable and highly elastic polyurethane hydrogels having a greater water content and a method for the production of the same.
In the production of polyurethane hydrogels, it is known to utilize polyethylene glycol having a molecular weight within the range of 1,000 to 20,000, as disclosed in Japanese Patent Publication No. 25,988 of 1971. It is also known to utilize for the production of polyurethane hydrogels, polyglycol ethers such as stearyl alcohol, ethylene glycol, trimethylolpropane and pentaerythritol, as disclosed in U.S. Pat. No. 2,948,691. In these known arts, polyethylene glycol or polyglycol ether is reacted with a polyisocyanate and the reaction product is then reacted with an excess amount of water to form a polyurethane hydrogel. However, the production of polyurethane hydrogels utilizing the above-mentioned known materials involves many disadvantages and cannot be successfully put into practice on an industrial scale. For example, in any of the cases where polyethylene glycol of a molecular weight of 4,000 is used and where ethylene glycol ether of polymethylolpropane having a molecular weight of 4030 is used, the polyurethane hydrogel obtained by mixing the reaction product with an excess amount of water is inferior in hardness and unstable. Particularly, if the water content of the hydrogel is over 95% by weight, the gelation cannot be carried out homogeneously and the product hydrogel tends to decompose to separate water from the gel. One of the reasons why the prior arts can only produce soft and unstable gel is the fact that polyethylene glycol or ethylene glycol ether is singly used. If polyethylene glycol is singly used, the greater the length of polyethylene glycol chain, the higher the solidifying point of polyethylene glycol. For example, with the molecular weight of polyethylene glycol per hydroxyl group being 1,000 or more, polyethylene glycol will be solidified even at room temperature. As a result thereof, the prepolymer obtained by the reaction between such polyethylene glycol and a polyisocyanate usually has a higher solidifying point and it will be easily solidified in winter. Accordingly, in order to use such the prepolymer having a higher solidifying point to form polyurethane hydrogel upon the reaction with water, it is necessary to take some preparatory steps. For example, the prepolymer must be liquefied by heating or it must be diluted with a suitable solvent to lower its solidifying point. In case of using a solvent for lowering the solidifying point of the prepolymer, another problems will arise. For example, if an inflammable solvent is used, care must be taken in handling the prepolymer to prevent it from catching fire. In addition, most of solvents pose problems in terms of poisoning and environmental pollution. Furthermore, in some cases addition of solvents causes decomposition of the hydrogel, reduction of the gel hardness and shrinkage of the hydrogel.
The primary object of the invention is to provide an improved polyurethane hydrogel in which the above mentioned disadvantages with conventional techniques can be avoided.
Another object of the invention is to provide an improved polyurethane hydrogel having a greater water content.
A further object of the invention is to provide an improved polyurethane hydrogel which is highly stable and highly elastic.
A still further object of the invention is to provide a prepolymer which is capable of forming a stable polyurethane hydrogel upon the reaction even with a corrosive electrolyte solution.
One of the other objects of the invention is to provide a prepolymer which is capable of forming a stable polyurethane hydrogel upon the reaction with an aqueous solution or dispersion containing detrimental and poisoning substances to entrap them therewith for the purpose of avoiding environmental pollution.
The other objects and advantages of the invention will become apparent from the following detailed description of the invention.